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"C" Shaped Building

"C" Shaped Building

"C" Shaped Building

(OP)
I am studying a "C" shaped building.  From the exterior, the building appears to be a rectangular box, 50' wide and 120' long.  Along the 120' length, the back wall is solid 8" CMU and along the front wall, 60' of the 120' is solid 8" CMU.  The 50' wall to the left is 8" CMU, but to the other end of the building, this 50' width is open (storefront).  I do not believe this is a stable design, as nothing prohibits the building from racking at the opposite end of the building from the 50' solid CMU wall.

Do you guys agree?

RE: "C" Shaped Building

No, I believe it is stable.   

Arguing with an engineer is like wrestling with a pig in mud. After a while you realize that they like it

RE: "C" Shaped Building

What type is the roof? How it is supported/attached to the walls? is the building located in seismic/storm active region? More info could make it more interesting.

RE: "C" Shaped Building

I don't think you have a stability problem.  The diaphragm keeps the end from racking at the open wall.  Maybe, if you have a truly flexible diaphragm, then you might have a problem.  No diaphragm is that flexible, though.

What you have is a 3-sided torsion box - nothing wrong with that.

RE: "C" Shaped Building

If you are using the IBC and looking a wood diaphragm it does not work per the IBC 2003 or the IBC 2009 reference AF&PA SDPWS 2008. Per the following IBC 2003 section 2305.2.5 Rigid diaphragms. "Design of structures with rigid diaphragms shall conform to the structure configuration requirements of Section 9.5.2.3 of ASCE7 and the horizontal shear distribution requirements of Section 9.5.5.5 of ASCE 7.
Open front structures with rigid wood diaphragms resulting in torsional force distribution are permitted provided the length, l, of the diaphragm normal to the open side does not exceed 25 feet (7620 mm), the diaphragm sheathing conforms to Section 2305.2.4, and the l/w ratio [as shown in Figure 2305.2.5(1)] is less than 1.0 for one-story structures or 0.67 for structures over one story in height. Exception: Where calculations show that diaphragm deflections can be tolerated, the length, l, normal to the open end is permitted to be increased to a l/w ratio not greater than 1.5 where sheathed in compliance with Section 2305.2.4 or to 1.0 where sheathed in compliance with Section 2306.3.4 or 2306.3.5." The AF&PA SDPWS 2008 is similar to this.

Garth Dreger PE
AZ Phoenix area

RE: "C" Shaped Building

It is impossible from your description to know whether the building is stable or not.  It can be, if a roof diaphragm or roof bracing is adequate to distribute the global torsion to the front and rear walls.  But even if stable, there could be deflection issues along the storefront side.

RE: "C" Shaped Building

I agree with hokie66 in that more data is required to give a resonable even gross assessment. You don't say anything about the quality of the roof, nor by whatever means the structure is thought to restrain torsion coming from  lateral forces passing through the center of the building, that apparently develop, according to your description. Depending on the code and region, these designs are not acceptable and well, that is what one first would examine ... is the structural setup and configuration permitted in the area?  

RE: "C" Shaped Building

(OP)
I quickly tapped-out the info to start this thread....here's more information.

The building is in a 100 mph wind zone.  The roof is comprised of metal roof deck that is screw fastened.  I do not know the deck fastening pattern, although I can about guess its something like a 36/4 pattern with (3) sidelap fasteners per span.

The load in the roof diaphragm is 38 kips, based on the 1991 code under which the building was built.  This results in a force along the end 50' shearwall of about 760#/Ft (not including any global rotational forces due to torsion).  

To begin with, the metal deck cant develope the 760#/ft diaphram capacity.  Secondly, even if it could, I believe this existing design is very poor in that it allows the opposite end of the building (opposite the 50' shearwall) to be too flexable....unfortunately, I dont know that that movement can be calculated....but may just be a "feel" sort of thing.  Also, there is NO direct connection of the metal deck to the 50' shearwall....the ONLY thing connecting the roof framing to the 50' shearwall is the joist bridging....which takes place (4) times along the 50' wall.

RE: "C" Shaped Building

If there is no connection between the diaphragm and the 50' shearwall, then yes, the building is unstable.

RE: "C" Shaped Building

I guess I've never tried to get a 3-sided shearwall system to work when the missing shearwall is one of the short walls.

Yes to this:
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No to this:

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RE: "C" Shaped Building

(OP)
Exactly JAE.  The one you say "no" to is exactly what I have, but, the long wall on the south side is only half as long as the long wall on the north side....as the rest of the south side is framed with beams and columns that do not contribute to the lateral stability of the building.

And like I say, the connection to the west wall.....there is no connection....except for the joist bridging termination, which I dont count as developing the diaphragm capacity.

Worse yet, the building owner is telling me, "well, its been there 18 years, thru Hugo and a tornado...and now you tell me its no good? etc etc"  I told him all I can do is report the numbers...its objective, not subjective.

Also, if I have to retrofit this building....I think it'll have to meet the 2006 IBC, not the 1988 SBCCI...which will require the CMU to be reinforced due to seismic....which cant be done, as the top of the existing wall has a bond beam at +16' that will prohibit the existing walls from being reinforced.....and I dont think I can reinforce the CMU walls externally, as the reinforcing requirements to meet the IBC code minimums for seismic are intended to create ductility in an otherwise brittle system.  I fear I may have to tell the client the building will need to come down.

RE: "C" Shaped Building

I could be wrong on this, and have no love for this layout, however the building has stood...survived...as the owner's claim, assume it is true, I couldn't stop to guess the flexible roof has made it work, and the beams, columns take the majority of load rather than the walls. Just thinking from another angle.

RE: "C" Shaped Building

I think the building is intrinsically stable, horrible arrangement but it works if the walls are good for the load. Before the roof can rotate about its vertical axis, the walls must fail in shear as well as warp out of plane.

Build a model out of cardboard or at least do a cutout of the roof and place it over the plan and see how the edges move off the foundation. You will see that before they can warp, the walls have to fail in in-plane horizontal shear.

Michael.
Timing has a lot to do with the outcome of a rain dance.

RE: "C" Shaped Building

(OP)
Here's a wildcard to all of this.....The perimeter CMU walls, that we've been discussing as shearwalls, are also load bearing.  The roof is joist and metal deck roof that clearspans 50', perimeter wall to perimeter wall.  The point to all of this is that there's about 810 lbs/ft of vertical load on the wall, the joists bear at +16', the wall is subject to 100 mph wind (80 mph per the old 1988 code) and is unreinforced.  The walls also have a 4' parapet above the +16' elevation....which oddly enough is reinforced...but only down to the roof bearing elevation...ie the parapet r/f is truncated at the point of max parapet bending!!

My calcs reveal the wall will not come close to working, as tensile stress in the block/mortar/faceshell, when subject to lateral wind, is about 107 psi...allowable per ACI 530 is 25 psi.  Compressive stress is ok.  

The problem here is that the local building official is telling me that this building will have to meet the requirements of the current code now....not the 1988 code.

I believe I'm stuck here.

RE: "C" Shaped Building

Well you may add an inner steel structure and if necessary pass even the wall weight to it. If there's no way that the existing walls can be accepted even with such works, it is likely better start anew, from a mere structural viewpoint. But the structure is a small part of the value of a building with a bussiness within. Hence one would try to find a way for the building be amended to what required. I would refrain from recommending demolition if there is an economically convenient way of keeping it. Anyway would report first on the feasibility, then on the cost, and ask the owner what he thinks.

RE: "C" Shaped Building

What was the reason that triggered the study? Renovation? Then on what? Agree with ishvaaag to preserve the  recommendation for demo as the last resort. There may exist creative way to strengthen this building if the owner willing to pay for it. But also agree, it seems a big chellenge with lots of headaches.  

RE: "C" Shaped Building

marinaman,

When you said the roof is metal deck, screw fastened (rather than welded), to me that suggested a metal roof cladding rather than a structural diaphragm deck.  If this is the case, with fasteners through the crowns, you can't use the roofing as a diaphragm.

You could probably reinforce the walls by cutting slots, inserting lapped bars in sections, and grouting.  You would have to saw out and chip away sections of the bond beam as well.  If all the parapet cores are not filled, you could use the empty ones to make the reinforcing continuous.

Not a nice task you have there.  The building official may be doing his job now, but the ball was dropped when the building was built.

RE: "C" Shaped Building

I can smell fish,

I would like to know how the exciting building looks. with a fully unreinforced wall, and no cores grouted, the hand of god must be holding it up. Are you sure we aren't missing something like steel column in the walls, these wouldn't be able to be located with the old tapping method, or something just as weird. Maybe you have a building not build on normal customs, sucht hat there are strange things going on, like maybe the walls have reo in them, but were never grouted.

As for the Shape of the c I have no problem either way as long as the walls and roof can take the loads, and deflections are controlled.  

Arguing with an engineer is like wrestling with a pig in mud. After a while you realize that they like it

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